An eye is one of the most complex organs in the human body. The sclera of the eye is the white surface visible around the cornea. The iris is the pigmented part of the eye surrounding the aperture that is the pupil and adjusting the pupil size to control the amount of light entering the eye. The cornea, which allows light to enter and focuses the light as it enters the eye, is the clear front surface of the eye covering the iris and the pupil. The crystalline lens is behind the pupil and further focuses light. Light that reaches the retina, which is an optical sensory membrane in the back of the eyeball, is converted into electronic signals, which in turn are transmitted via optic nerve to the visual cortex in the brain to be interpreted as sight.
Due to its complexity and sensitivity, each individual eye is prone to a host of injuries and disorders. Ocular higher-order aberrations in the cornea, for example, may originate from a corneal dystrophy (i.e., a hereditary disorder), inflammation in the eye, surface abrasions, burns, and other trauma. Corneal ectasia is an umbrella term for a group of conditions that cause a progressive thinning and distortion of the corneal shape, including keratoconus, keratoglobus, pellucid, marginal degeneration, Terrien's marginal degeneration, post-LASIK (laser-assisted in situ keratomileusis) treatment, post- radial keratotomy, post-penetrating keratoplasty, and post-cornea transplantation or grafting.
While lower-order aberrations include common aberrations like defocus (e.g., nearsightedness and farsightedness) and regular astigmatism, higher-order aberrations may include secondary astigmatism, spherical aberration, coma, trefoil, and quadrafoil stemming from irregular deformations, stress lines, and scarring on the corneal surface. For example, keratoconus may change the corneal side profile from a normal gradual curve to a steeper curve to a pronounced conical shape. These structural irregularities may refract, reflect, and/or absorb light when it reaches the cornea, thus distorting (by causing, e.g., blurriness, ghosts, halos, starbursts, and loss of contrast) and sometimes multiplying the image received by the retina (diplopia or polyopia). Vision in low light or at night may be even more difficult because the pupil dilates to expose and receive light through even more of the irregular corneal surface. Other symptoms associated with ocular higher-order aberrations include eye strain, itching, pain, and photophobia (i.e., sensitivity to bright light). Patients with ocular higher-order aberrations may experience temporary or permanent visual distortions, and for some, their vision may progressively deteriorate.
In particular, the abnormal degenerative disorder of keratoconus, in which the cornea both steepens and thins, produces a large magnitude of higher-order aberrations, around 5-6 times typically found in normal eyes, thus severely degrading retinal image quality. For a 5.7 mm pupil, Guirao et al. theoretically demonstrated that an improvement by a factor of 12 in retinal image contrast at 16 cycles per degree (c/deg) could be achievable in an eye with keratoconus compared to only a 2.5-fold benefit in a normal eye. Similarly for a 6 mm pupil, when computing the area under the modulation transfer function, Pantanelli et al. estimated a 4.4-fold improvement in retinal image quality in an eye with keratoconus compared to only a 2.1-fold benefit in a normal eye. Therefore, an eye with keratoconus stands to benefit to a great extent by correcting higher-order aberrations.
In 1961, Smirnov theorized that upon quantifying the ocular higher-order aberrations of an eye it was conceivable to make ophthalmic lenses to neutralize the aberrations; however, he also conceded that this was highly impractical given the laborious nature of the aberration measurements.
Existing attempts to correct ocular higher-order aberrations and to treat underlying causes like corneal ectasia include soft contact lenses, hybrid contact lenses, rigid gas-permeable contact lenses, and conventional scleral lenses. However, these existing lenses help only a small percentage of patients and only to a limited extent. For example, both Sabesan et al. and Marsack et al. attempted to correct higher-order aberrations with soft contact lenses by incorporating centration information and accounting for the effect of the interaction of the ophthalmic lens with the ocular surface. Even then, the residual higher-order wavefront error was still 0.93±0.19 μm for a 6 mm pupil in the Sabesan study. Similarly, the residual higher-order wavefront error was 0.31 μm and 0.38 μm for a 4.25 mm pupil in two keratoconus patients and 0.76 μm for a 4.5 mm pupil in a third keratoconus patient in the Marsack study.
Moreover, some patients have corneal scarring, recurrent corneal erosions due to poor-fitting contact lenses, and/or an intolerance to contact lenses. Although conventional scleral lenses minimize contact with the cornea by having a large diameter and a vaulted structure over the cornea, even these lenses only correct for lower-order aberrations and some higher-order aberrations but not residual higher-order aberrations originating from the posterior corneal surface, the crystalline lens (which may develop an aberration to compensate for a corneal aberration), and/or the scleral lens itself if the lens is decentered (e.g., due to a lack of individual patient customization).
When lenses fail to correct ocular higher-order aberrations and/or to treat other associated symptoms, clinicians may resort to more invasive procedures like corneal transplantation surgery, whereby a damaged or diseased cornea is replaced by donated corneal tissue. Together, keratoconus and post-refractive corneal ectasia are the second most frequent indication for corneal transplantation, accounting for about 15% of the corneal transplantations performed in the United States. In addition to common dangers associated with ocular surgery like infection, inflammation, injury, visual impairment, and temporary or permanent blindness, corneal transplantation carries both short-term and lifelong risks of corneal graft failure and rejection.